Spark plasma sintering of Ti-diamond composites

Spark plasma sintering of Ti-diamond composites

Ceramics International 45 (2019) 11281–11286 Contents lists available at ScienceDirect Ceramics International journal homepage:

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Ceramics International 45 (2019) 11281–11286

Contents lists available at ScienceDirect

Ceramics International journal homepage:

Spark plasma sintering of Ti-diamond composites Awadesh Kumar Mallik




, Mitun Das , Sumana Ghosh , Dibyendu Chakravarty




CSIR-Central Glass & Ceramic Research Institute, Kolkata, 700032, West Bengal, India Institute for Materials Research (IMO), Hasselt University, 3590, Diepenbeek, Belgium IMOMEC, IMEC vzw, 3590, Diepenbeek, Belgium d International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, 500005, Telangana, India b c



Keywords: Titanium matrix composites Spark plasma sintering Raman spectroscopy X-ray diffraction Hardness

Laser melting of Ti-diamond powders have been found to enhance the mechanical properties of technologically important material like titanium matrix composite (TMC). However, there is a tendency for the diamond to graphitise during the laser melting process. In order to overcome this fallacy, an alternate processing route, namely, spark plasma sintering (SPS) was adopted for fabricating the TMC's. A wide range of powder compositions varying from 5 to 50 wt percentage of diamond (0.25 μm) was added to titanium and the as-sintered compacts were investigated by X-ray diffraction (XRD), Raman spectroscopy, Scanning Electron Microscope (SEM), and Energy Dispersive Spectroscopy (EDAX). In-situ phase changes were observed with increase in diamond content in the composition. Addition of diamond upto 15% led to formation of a mixed Ti and TiC phase in the matrix. Interestingly there was no trace of metallic titanium with 20% diamond in the composition and a TiC-only phase was observed, corroborated by an abrupt increase in hardness to 1076 Hv. At even higher diamond percentages there was trace of unreacted carbon along with TiC. This work indicates, for the first time, the use of SPS as an alternate route for fabricating in-situ TMCs with enhanced mechanical properties.

1. Introduction TMCs are technologically important composite materials [1–4] for their superior thermal [5] and mechanical properties. Metallic Ti is ductile but not hard enough for mechanical applications. Addition of hard ceramics like diamond significantly improves the mechanical properties like hardness and Young's Modulus of TMC's [6–9]. However, there is a tendency for diamond to graphitise during laser assisted melting of the Ti-diamond composite powder leading to deterioration of the mechanical properties. An attempt has been made in this work to overcome this limitation by adopting a completely novel and extremely fast powder metallurgy route, namely, spark plasma sintering (SPS) [10–13] for developing such TMCs. The mechanism of SPS and the specific roles of DC pulsed current, applied pressure and other processing parameters on material properties are well documented. SPS is also an established technique for consolidating carbon based composites, including CNT's and graphene, into dense compacts having superior mechanical properties [14–16]. There are also a few reports on sintering of diamond by SPS [17–20]; however, there are no specific reports on phase evolution and mechanical properties of Ti-diamond composites by SPS. As the laser melting of Ti-diamond powders

improved the properties of the end product, we envisaged SPS to have a similar or possibly a better effect on the properties due to the short sintering cycles and inherent mechanisms involved in the processing. The present investigation was undertaken to evaluate the sinterability of Ti-diamond composites by SPS and to investigate the gradual phase development and the resultant physical and mechanical properties with diamond content in the composition. The physical and microstructural properties of the composites after sintering were characterized by XRD, SEM, EDAX and Raman investigations, whereas, the Vicker's indentation technique yields the hardness values of the resultant TMCs. 2. Materials & methods 2.1. Powder preparation The Ti powders were procured from ATI Powder Metals, USA and diamond powders were procured locally from Eastern Diamond Products Pvt. Ltd., Kolkata. Fig. 1a and b shows the typical x-ray diffraction patterns of the as-received Ti and diamond powders, respectively. The average particle size of the starting Ti powders was around 50–150 μm, as observed from the SEM micrograph Fig. 1c. The

Corresponding author. CSIR-Central Glass & Ceramic Research Institute, Kolkata, 700032, West Bengal, India. Corresponding author. E-mail addresses: [email protected], awad[email protected] (A.K. Mallik), [email protected] (D. Chakravarty).

∗∗ Received 19 December 2018; Received in revised form 11 February 2019; Accepted 27 February 2019 Available online 01 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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Fig. 1. X-ray diffraction patterns (a and b) and scanning electron micrographs (c and d) of the as-received starting powders.

corresponding average particle size value for the diamond powder was 1–4 μm, as shown in Fig. 1d. The titanium powders were thoroughly mixed with different weight percentages of diamond powders using a turbula mixture to develop well dispersed powder mixtures for SPS. For the present study titanium was used as the matrix with varying amounts of diamond ranging from 5 to 50 wt%. The different compositions and corresponding SPS conditions used in the study are shown in Table 1. The nomenclature “Dx” used for designating the different sample compositions, where ‘x’ indicates the weight percent of diamond in the composition. Thus, the composition with 85% Ti and 15% diamond is designated as D15. 2.2. Spark plasma sintering (SPS) Spark plasma sintering was performed under partial vacuum of 8 Pa Table 1 Ti-diamond composites synthesized using SPS at different processing conditions. Sl. No.

Composition (wt. %)/Nomenclature

SPS condition (Temp, oC/Pressure, MPa)

1 2 3 4 5 6 7 8 9 10 11


1500/50 1450/50 1400/50 1400/80 1450/80 1450/50 1450/50 1450/50 1450/50 1400/50 1350/50

Ti(50)-Diamond(50)/D50 Ti(60)-Diamond(40)/D40 Ti(70)-Diamond(30)/D30 Ti(80)-Diamond(20)/D20 Ti(90)-Diamond(10)/D10 Ti(95)-Diamond(5)/D5

in a Dr. Sinter 1050 apparatus (SPS Syntex Inc. Tokyo, Japan) having pulse duration 3.3 ms and on-off pulsing ratio of 12:2. This sequence denotes the time frequency for which the pulsed current is applied followed by a period where no current is applied. The sintering temperature used for the current set of samples ranged between 1350 and 1500 °C, and the imposed stresses were 50 or 80 MPa applied before the sintering commenced and was held constant throughout. The samples were heated at a rate of 150–175 °C/min from room temperature to the final temperature and held isothermally for 5 min. The voltage varied between 3 and 4 V and the applied current flow was between 600 and 2000 amps. Temperatures were recorded by a radiation pyrometer focused on a hole at the die wall. 5 g of powders were loaded into cylindrical graphite dies of 20 mm inner diameter which were covered with 0.2 mm thick graphite foil for proper contact between the die wall and the powder; thickness of the final sintered samples were ∼ 3–4 mm which were ground and polished thoroughly before characterization. The total densification was measured using an LVDT which yielded the cumulative value of the sample, graphite moulds and the loading systems. 2.3. Characterisations An SEM (Phenom proX, Netherlands) equipped with energy-dispersive spectrometer (EDS) was used to look at the composite microstructure and the elements present therein. Constituent phases in the composites were determined by an X'Pert Pro MPD diffractometer (PANalytical) operating at 45 kV and 40 mA with Ni filtered CuKα radiation. Raman spectrometer (STR500, Cornes Technologies) was used for identifying the different carbon phases present in the composite. The microhardness was measured at a load of 500 g for 15 s (ESEWAY, W4303, UK).


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to cubic diamond phase at 44° and 75° due to (111) and (220) plane reflections (similar to Fig. 1b for the as-received diamond powders). D50 produced TiC phase peaks as found in other sintered samples, but in addition there is also a small peak at 76° due to the reflection from (222) planes. But there are no peaks corresponding to pure Ti in D50. It appears that all the Ti has reacted with carbon to form TiC phase. In addition, there is also some unreacted diamond peaks. To summarise the XRD peak results, initial addition of diamond in the Ti metal matrix produced a mixture of Ti and TiC phases but as the percentage was increased TiC-only phase was obtained. Any further addition of diamond could not make all the carbon react with Ti as can be found from the remnant diamond phase in the D50 sample. 3.2. Raman spectroscopy

Fig. 2. XRD analysis of SPS samples with varying percentages of diamond in Ti matrix.

3. Results and discussion 3.1. Phase analysis from XRD The peaks are mixtures of Ti and TiC phases as shown in Fig. 2. From the initial composition of 5% diamond (D5) to 10% diamond (D10) in Ti metal matrix there is a gradual shift of peak predominance from metallic Ti to titanium carbide. But with increase upto 50% diamond powder (D50), peaks of diamond could also be observed in the sintered samples. Surprisingly, there was no XRD peak corresponding to the graphite phase. Sample D5 gave the most intense peak at 39.97° from the (101) plane reflection of Ti (similar XRD peaks to the as received powder Ti samples in Fig. 1a). The second and the third most prominent peaks are at 36° and 41.83° from (111) and (200) plane reflections of fcc TiC; another small peak corresponding to TiC is at 60.75°. There are other less intense Ti peaks at 38.21°, 34.91° and 52.76° due to reflections from (002), (100) and (102) planes. For sample D10 the highest intensity peak corresponding to TiC phase appears at 36.22° from the (111) plane reflection. The second most intense peak is at 40.19° from pure Ti (101) plane. Another Ti (002) peak with less intensity was detected at 38.43°. D10 sample also shows TiC peaks similar to D5 at 2θ values of 36° (111), 41° (200) and 60° (220). A new peak position corresponding to (311) plane reflection for the TiC phase also appeared at 72.86°. The sample D20 with 20% diamond content did not yield any XRD peak for pure Ti; all the identified peaks in Fig. 2 for the sample D20 are for TiC phase. Binary phase diagram has been reported by many authors for Ti-C system. It has been shown that with the small percentages addition of carbon into the Ti metal matrix the sintered product becomes a mixture of pure Ti and TiC phases. But as the percentage of carbon is increased, the resultant phase becomes purely TiC. Such TiC-only phase was produced at 20% carbon (diamond powder) addition in the present work as shown in Fig. 2. The phase diagram also predicts that if the carbon percentages varies from 40% to 50% then quantifying the structure and composition of TiC phase becomes difficult [4]. In the present study, 50% diamond was added into the Ti metal matrix and it was found that all the diamond powder could not react with Ti metal to form TiC-only phase. There is the presence of XRD peaks corresponding

The first batch of samples in SPS was prepared by mixing 15% diamond powder with 85% Ti powder. There are two processing parameters which could affect spark plasma sintering of the composite powders, namely, temperature and pressure. It was found by experimentation that optimum densification without melting could be achieved at 1450 °C and 50 MPa pressure. The Raman spectroscopy from the as-sintered sample produced graphite-only peaks (disordered (D) and crystalline (G) graphite peaks with simultaneous appearance of G’ second phonon line at 2700 cm−1) as the powders were sintered inside a graphite mould, as shown in Fig. 3a. In order to remove the outer graphitic layer, the sample was ground to half its thickness by surface grinder machine. The optical image, Fig. 3b, of the SPS sample after grinding appears to consist of two contrasting regions. Raman signals from the whitish region (Fig. 3c) did not produce any carbon peak. It might be that the white region is a composite region of titanium and titanium carbide. On the contrary, the black region produced both diamond and graphite peaks. Natural diamond gives Raman signal at 1332 cm−1but in Fig. 3d the diamond peak appeared to be shifted to 1334 cm−1. The 2 cm−1 shift corresponds to a compressive stress of 1.13 GPa (The residual stress (γ) is calculated in GPa using the equation, γ = −0.567 × δν, where δν is the peak shift in cm−1 [21]). The shift from the theoretical peak position might be due to the 50 MPa pressure applied during spark plasma sintering leading to compressive stress in the sintered samples. Crystalline graphite peak (G) could also be observed at 1583 cm−1 which indicates the conversion of some of the diamond into graphite possibly due to high temperature and current being used during SPS. But no graphitic peak from the white region confirms full consumption of carbon in forming TiC phases alongside Ti metal matrix as found in X-ray diffraction peaks, Fig. 2. After optimising the SPS conditions for optimum densification without melting at the 15% diamond composition (D15), the percentages of diamond in the Ti matrix was varied from 5% to 50% for further investigations. Raman spectra from four such samples sintered at 1450 °C and 50 MPa pressure is presented in Fig. 4. It is found that the spectra for samples D5, D10 and D50 are identical with that from the black region of D15. All the samples are having diamond peak around 1332 cm−1 and crystalline graphite peak at around 1580 cm−1. Surprisingly, D20 sample does not produce any Raman peak which can be attributed to any form of carbon. It can be inferred that all the added diamond might have been consumed by the Ti metal to form TiC phase in agreement with previous reports [4]. The XRD peak for D20 is also in agreement with Raman data for D20 sample. The Raman and X-ray data for D50 sample are concurring with the fact that both show the presence of remnant diamond in the sintered sample. XRD peak of D50 does not have any graphite peak whereas the Raman spectra has signature of crystalline graphite peak at 1580 cm−1. It is to be noted that graphite is 50 times more sensitive than diamond in scattering Raman


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Fig. 3. (a) Raman signals from the as prepared SPS sample; (b) Optical image of the SPS sample after grinding off the outer graphitic layer; (c) Raman signal from white region of Fig. 3b; (d) Raman signals from the black region of Fig. 3b.

signals [21] and because of such sensitive nature of the graphitic carbon D50 produces Raman signal but does not show any XRD peak. D50 also does not have any XRD peak corresponding to pure metallic titanium. 3.3. Microstructural analysis Fig. 5a–f are ondary electrons The samples are sponding optical

the electron microscope images produced from secand corresponding EDAX signals from SPS samples. showing two distinct regions similar to their corremicroscope image Fig. 3b. D5 sample has scattered

Fig. 4. Raman signals from the samples with varying percentages of diamond in Ti powder.

rounded dark spots (Fig. 5a), which on increasing amount of diamond powder in Ti composition (D10: Fig. 5b and D20: Fig. 5c), starts to interconnect. D20 has just the reverse characteristics SEM micrograph than D5 - the entire surface has become covered by darker phases. The EDAX of such contrasting white region (D10 sample - Fig. 5f) shows that the whitish region is consisting of 100% Ti (metal phase), whereas the electron diffraction peaks from its darker region shows that the microstructure is consisting of 70.5% Ti and 1.5% carbon (carbide phase). So it can be inferred that the composite microstructure consists of a mixture of Ti (white) and TiC (dark) phases, which has already been indicated by the XRD peak for D10 sample in Fig. 2. Initial occurrence of both the white and darker region due to the presence of both the Ti and TiC phases gradually becomes TiC only darker phase in D20 sample, as evident from Fig. 5c and inset images. Surprisingly the EDAX signal from the darker phase (TiC region in Fig. 5f) shows 28% of atomic oxygen for the sample D10. The samples were polished down to near-mirror finish surface before SEM study. Moreover the polished surface was treated with etchant in order to make the grain boundaries visible. So the sample surface might have been oxidised, giving rise to 28% oxygen signals. Now the XRD does not show any oxide phase of Ti as it was done before etching for SEM sample preparation. Fig. 5e shows the surface of the D50 samples. The average particle sizes of Ti were 50–150 μm and diamond powders were of 1–4 μm, Fig. 1c and d. The rounded spot sizes are in agreement with the initial particle size of Ti metal powders. EDAX in Fig. 5e shows the formation of TiC phases by diffusion of carbon into Ti white metal matrix, but black/darker region is completely made up of carbon (also evident from XRD diamond peak). There is no Ti metal-only region for D50 sample, which was present for D5 or D10 samples. It appears that D20 sample onwards, there is only TiC phase, as already confirmed by XRD, Raman data.


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Fig. 5. SEM images at 1500X magnification of: (a) D5, (b) D10, (c) D20, (d) D50; EDAX signals (e) from D50 sample surface and, (f) from D10 sample surface.

3.4. Mechanical property

4. Conclusion

Hardness was found to increase with gradual increase in diamond content in the Ti matrix from 5 to 10% (see Table 2). It has already been observed from XRD and SEM images that addition of diamond results in formation of binary microstructure of metallic Ti and hard carbide phase. The increased hardness of D5 and D10 compared to pure Ti is an indication of that. However, 20% addition of diamond resulted in an abrupt increase in Vicker's hardness to 1076 HV. The cross section of D20 does not show any graphitic or diamond peak in the Raman spectroscopy. It is confirmed that all the Ti has been consumed to form hard TiC ceramic for the D20 sample. Li et al. [8] prepared TiC composite coatings by mixing different percentages of carbon nanotube powders with Ti metal powders. They found that with increasing percentages of CNT the microhardness (HV0.5) of the composite coating increased from 187 for pure Ti metal to 1125 for 20% CNT addition [22]. The present work is in exact agreement with their finding that 20% addition of diamond powder (D20) could greatly enhance the Vicker's hardness of the SPS composite sample.

An attempt has been made to synthesise a technologically important class of material titanium matrix composite with diamond as filling material. This is the first such effort to sinter the material by spark plasma sintering route. The sintered product has found to be consisted of mixed phases of Ti metal, TiC ceramic and there was gradual emergence of unreacted diamond in the samples with higher percentages of mixed diamond powders in the TMC compositions. Sintering of diamond powder has the risk of graphitisation but the x-ray diffraction peaks do not reveal presence of any graphite. On the other hand, the Raman spectrum shows graphitic bands. It was because of the graphite mould that was used during SPS in the as-prepared samples. But even after surface grinding of the as-prepared samples, the graphitic peaks were also observed in some of them, which may be due to oversensitivity of graphite than diamond for Raman scattering. Specifically, the D20 sample neither had diamond nor had graphitic Raman peaks. It was concluded that the carbon from added diamond powder fully reacted with Ti metal to form hard TiC ceramic phases. With further addition of diamond in more than 20% weight percentages in the TMC composition, there was emergence of unreacted carbon as shown by the XRD, Raman and SEM results. This work shows for the first time a novel process for fabricating titanium matrix composites using the powder metallurgy route to yield components having improved mechanical properties.

Table 2 Hardness of sintered samples. Sample

Vicker's Hardness (HV) 1 Kgf, 15 s dwell time

D5 D10 D20

327 ± 4 370 ± 7 1077 ± 100


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Acknowledgement DC acknowledges Department of Science and Technology, Govt. of India for the spark plasma sintering facility at ARCI, Hyderabad. AKM is presently on Extraordinary Leave (EOL) from CSIR-CGCRI and working as FWO postdoctoral researcher at IMO-IMOMEC, University of Hasselt, Belgium (Project no. R - 9115). References [1] W. Liu, J.N. DuPont, Fabrication of functionally graded TiC/Ti composites by laser engineered net shaping, Scripta Mater. 48 (2003) 1337–1342. [2] W. Wanliang, L. Yong, Y. Dezhuang, H. Wenrong, Microstructure of TiC dendrites reinforced titanium matrix composite layer by laser cladding, J. Mater. Sci. Lett. 22 (2003) 1169–1171. [3] J. Tiley, S. Gopagoni, A. R. P. Singh, J. Y. Hwang, T. W. Scharf, and, R. Banerjee, reportMicrostructural Evolution in Laser Deposited Nickel-Titanium-Carbon in Situ Metal-Matrix Composites, January 2010, Interim Report, Air Force Research Laboratory, Materials and Manufacturing Directorate, United States Air Force.. [4] S. Gopagoni, Microstructure Evolution in Laser Deposited Nickel-Titanium-Carbon in Situ Metal Matrix Composite, Thesis for the Degree of Master of Science, University of North Texas, December, 2010. [5] S. Ren, X. Shen, C. Guo, N. Liu, J. Zang, X. He, X. Qu, Effect of coating on the microstructure and thermal conductivities of diamond–Cu composites prepared by powder metallurgy, Compos. Sci. Technol. 71 (2011) 1550–1555. [6] W. Liu, J.N. DuPont, Fabrication of carbide-particle-reinforced titanium aluminide–matrix composites by laser-engineered net shaping, Metall. Mater. Trans. 35A (2004) 1133. [7] C. Guo, J. Zhou, J. Chen, J. Zhao, Y. Yu, H. Zhou, Improvement of the oxidation and wear resistance of pure Ti by laser cladding at elevated temperature, Surf. Coating. Technol. 205 (2010) 2142–2151. [8] Q.H. Li, M.M. Savalani, Q.M. Zhang, L. Huo, High temperature wear characteristics of TiC composite coatings formed by laser cladding with CNT additives, Surf. Coating. Technol. 239 (2014) 206–211. [9] S. Roy, M. Das, A.K. Mallik, V.K. Balla, Laser melting of titanium-diamond composites: microstructure and mechanical behaviour study, Mater. Lett. 178 (2016) 284–287.

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